can: softing: drop 'channel' sysfs attribute

netdev->dev_id obsoletes this property.
None of the remaining properties contribute to udev detection methods.
The regular calls for the sysfs group can thus safely be restored.

Signed-off-by: Kurt Van Dijck <kurt.van.dijck@eia.be>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>